head to head
Rainbow Six Siege asks more than Tom Clancy's Rainbow Six: Vegas, and the gap is reads.

Reads · Grand Arena
A tactical breach-and-defend shooter driven by mind games over angles and gadgets.

Execution · Competitive PvP
Tactical gunfights testing aim, entry planning and prediction of human opponents online.
Rainbow Six Siege leans Reads (Grand Arena); Tom Clancy's Rainbow Six: Vegas leans Execution (Competitive PvP).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Rainbow Six Siege
Crisp aim required but TTK is unforgiving.
Tom Clancy's Rainbow Six: Vegas
Aim and snap-to-cover gunplay punish slow hands
Rainbow Six Siege leans clearly harder on reads.
Rainbow Six Siege
Defenders adapt holds; attackers fake and bait.
Tom Clancy's Rainbow Six: Vegas
Multiplayer angles and pushes demand reading human opponents
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Rainbow Six Siege
Site rotations only; loadouts deterministic.
Tom Clancy's Rainbow Six: Vegas
Terrorist hunt spawns vary modestly between runs
Rainbow Six Siege leans a little harder on optimization, though the gap is small.
Rainbow Six Siege
Operator picks, gadget placement, and utility usage.
Tom Clancy's Rainbow Six: Vegas
Room-entry planning and map knowledge are studyable
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.